The present invention relates to a method for determining the transit time of an ultrasonic signal from an ultrasonic flow sensor, and to an ultrasonic flow senor.
Ultrasonic flow sensors are used, in particular, to measure the volume flow, mass flow, or the flow rate of a gaseous or liquid medium flowing through a pipeline. A known type of ultrasonic flow sensor includes two ultrasonic converters located such that they are offset in the direction of flow, each of which generates ultrasonic signals and transmits them to the other ultrasonic converter. The ultrasonic signals are received by the other converter and are evaluated using electronics. The difference between the transit time of the signal in the direction of flow and the transit time of the signal in the opposite direction is a measure of the flow rate of the fluid.
FIG. 1 shows a typical design of an ultrasonic flow sensor with two ultrasonic converters A, B, which are located inside a pipeline 3 and are diametrically opposed at a distance L from each other. A fluid 1 flows in pipeline 3 with a velocity v in the direction of arrow 2. Measurement path L is tilted relative to flow direction 2 at an angle α. While a measurement is being carried out, ultrasonic converters A, B send ultrasonic pulses to each other. The signals are decelerated or accelerated, depending on the direction of the flow. The transit times of the ultrasonic signals are a measure of the flow rate to be determined.
FIG. 2 shows a greatly simplified schematic depiction of the electrical circuit of the system in FIG. 1. The two ultrasonic converters A, B are connected with control and evaluation electronics 4 and are activated by an oscillator with a specified clock frequency 8 (square-wave signal). Ultrasonic signals 15 generated as a result (only envelope 16 of ultrasonic signals 15 is shown here) travel along measurement path L and are detected by the other ultrasonic converter A, B. Transit time t12 or t21 of signals 15 is measured.
To measure the transit time of an ultrasonic signal 15, it is essential that the instant of receipt of ultrasonic signal 15 be determined unequivocally and exactly. Different events can be defined as the instant of receipt of an ultrasonic signal. It is known from the related art, e.g., to define the first zero crossing N0 of ultrasonic signal 15 after the signal amplitude has exceeded a specified threshold as the “instant of receipt”. As an alternative, e.g., the instant at which the maximum amplitude or centroid ts of envelope 16 of ultrasonic signals 15 occurs can be defined as the instant of receipt. It is also known to determine the transit time of ultrasonic signal 15 by evaluating the phase of the signal relative to reference timing signal 8. Conventional methods for determining transit time are typically relatively complex or they are not sufficiently robust against interfering signals.